Zeeman shift--a tool for assignment of 14N NQR lines of nonequivalent 14N atoms in powder samples.

The use of Zeeman perturbed 14N nuclear quadrupole resonance (NQR) to determine the ν+ and ν-14N lines in polycrystalline samples with several nonequivalent nitrogen atoms was investigated. The 14N NQR line shift due to a weak external Zeeman magnetic field was calculated, assuming isotropic distribution of EFG tensor directions. We calculated the broad line distribution of the ν+ and ν- line shifts and experimentally confirmed the calculated Zeeman field dependence of singularities (NQR peaks) in cyclotrimethylenetrinitramine (RDX) and aminotetrazole monohydrate (ATMH). The calculated and measured frequency shifts agreed well. The proposed measurement method enabled determination of which 14N NQR lines in ATMH belong to ν+ and which to ν- transitions.